Systems in this field are well known in the art. Original systems employ time domain techniques. The amplitude of a given signal is acquired at a fixed sampling rate. The sampling rate is generally many times higher than the highest frequency of the signal. The accumulated number of bits from such a sampling rate requires a large storage capacity and/or a fast transmission rate.
In order to decrease the required capacity various techniques have been devised for compressing the amount of information required to be transmitted in these time domain systems. Time domain compression techniques can result in systems which are complex and which tend to degrade the quality of the reconstructed signal.. Such systems are disclosed in U.S. Pat. No. 4,382,160 issued May 3, 1983 to Gosling et al; 4,630,257 issued Dec. 16, 1986 to White; 4,404,532 issued Sep. 13, 1983 to Welti; 3,973,081 issued Aug. 3, 1976 to Hutchins; and 3,621,150 issued Nov. 16, 1971 to Lyndeborough.
The system disclosed in the Gosling et al patent discloses methods and apparatus for encoding and constructing speech signals. The disclosure describes a system having an analog to digital converter sampling at rates of approximately 20,000 samples per second. This is two to ten times the Nyquist criteria but is necessary to ensure no useful information, is lost between samples.
The A to D converter is connected to two storage means which store the present sample and the preceding sample. A comparator compares the two samples to determine when there is a maxima or minima in the signal. The A to D converter also detects zero crossings on a datum line and signals such crossings by changes in its plurality bit.
A first counter counts the number of maxima and minima between consecutive zero crossings. A second counter counts the period of time elapsed between the consecutive zero crossings.
The time and number of maxima and minima for a period are compared to values in a look-up table and translated into a number representing a matching value. This number is transmitted and reconstruction is based on the transmitted number.
This system does not reconstruct the signal to have the maxima and minima in the positions in which they occurred in the original signal, but only reconstructs the signal to have the given number of maxima and minima between zero crossings. As well this system does not retain the amplitude information. Additionally, the signal is required to fit within certain parameters or else the size of the look-up table will become unmanageable. The reconstructed signal loses specific keys to intelligibility when dealing with speech signals and, more importantly, by the inventors own admission cannot be applied to signals outside of the speech field.
In order to further reduce the amount of information required to be stored or transmitted techniques in the frequency domain have been developed. Related patents include U.S. Pat. No. 4,622,680 issued Nov. 11, 1986 to Zinser; 4,086,174 issued Apr. 29, 1986 to Wong; 4,374,304 issued Feb. 15, 1983 to Flannigan; 4,086,431 issued Apr. 25, 1978 to Franssen; 4,034,160 issued July 5, 1977 to Van Gerwen; 3,667,074 issued May 30, 1972 to Iwasaki et al; and 3,499,996 issued Mar. 10, 1970 to Klayman et al. These techniques, while decreasing the amount of transmitted information, are susceptible to errors induced by background noise. Additionally their restricted bandwidth causes the loss of many of the keys to speech. Many of the systems would not be capable of handling the bandwidth requirements of systems outside the speech field.
To decrease both the susceptibility to noise and the amount of information required to be stored digital spectral domain systems have been developed. These systems include those which employ an analog to digital converter and determine the frequency of the signal by correlation to reference samples as disclosed in PCT publication No. W087/01542. This system runs into problems similar to those of the Gosling et al patent in that look-up tables or the like are employed.
Further systems employing digital spectral techniques include those employing fourier transforms or approximations thereof. Related patents. include U.S. Pat. No. 4,667,340 issued May 19, 1987 to Armand et al; 4,536,886 issued Aug. 20, 1985 to Papamichalis et al; and 3,681,530 issued Aug. 1, 1972 to Manley et al. These fourier transform systems typically sample the signal to be acquired with an A to D converter.
Fourier transforms require magnitude samples to be acc for the inverse of the required frequency resolution. The accumulated samples may be called a record. As is evident the length of a record will increase with increasing resolution requiring increasing storage capacity. Additionally the time delay in accumulating the record increases with increasing frequency resolution.
To determine the frequency components in a signal calculations are performed on the records according to a given fourier transform algorithm. The calculations are extremely computation intensive further adding to the time delay. The longer the record length the greater length of time the calculations will take.